In a backlight unit 169 in which fluorescent tubes 191 as shown in FIG. 20 are arranged throughout and which emits planar light, it is easy to change the distribution of brightness in a direction p in which the fluorescent tubes are aligned. For example, in the aligned fluorescent tubes 191, when the brightness of the fluorescent tubes 191 around the ends is set lower than that of the fluorescent tubes 191 around the center, a brightness distribution diagram (brightness distribution diagram specified by a brightness level L and the p direction) as show in FIG. 20 is obtained.
In the brightness distribution described above, the brightness of the planar light around the center that is relatively easily recognized by a visually recognizing person is high, and the brightness of the planar light in the perimeter thereof is low. However, in a human visual characteristic, uneven brightness of the planar light in the perimeter and with a low brightness, in particular, is not perceived. Hence, in the backlight unit 169 described above, part of the fluorescent tubes 191 is lower in brightness than the other fluorescent tubes 191, with the result that the power consumption is reduced.
However, in the backlight unit 169, since the distribution of brightness in a direction q in which the fluorescent tubes 191 extend cannot be changed, the power consumption is not sufficiently reduced. There has recently been a backlight unit in which LEDs (light emitting diodes) are arranged throughout in a matrix (for example, see patent document 1). In a liquid crystal display device incorporating such a backlight unit, planar light is partly controlled based on the result of analysis of data on an image displayed on a liquid crystal display panel.
This technology is called local dimming; only a part of planar light corresponding to a part of a display image on the liquid crystal display panel that has a relatively high brightness has a high brightness. Hence, this technology is effective for reducing the power consumption of a backlight, and therefore the power consumption of a liquid crystal display device.